1. Field of the Invention
The present invention relates to an exhaust emission control device for an internal combustion engine, and more specifically to the technology of regenerating a particulate filter for capturing particulate matter contained in the exhaust gases.
2. Description of the Related Art
The exhaust gases emitted from a diesel engine mounted on a bus, truck and the like, include a large quantity of minute particulate matter (hereinafter abbreviated as PM), besides HC, CO, NOx, etc. Therefore, as a postprocessing device of a diesel engine, a diesel particulate filter (hereinafter abbreviated as DPF ad libitum), which has a configuration in which after the PM is captured, the trapped PM is directly incinerated by an external heat source, such as a burner, has been developed and come into practical use.
For instance, there is disclosed a device having a configuration in which a combustion heater is utilized to increase the temperature of exhaust gases and further the temperature of a DPF by combustion heat, and at the same time a combustion gas or vaporization fuel produced by the combustion heater in a high exhaust temperature is supplied to an exhaust emission control device (for example, a DPF having an oxidizing function) as an additive to eliminate the PM by reaction heat generated on the DPF (for example, Unexamined Japanese Patent Publication No. 2000-186545).
There has been recently developed a continuous regeneration-type DPF in which an oxidation catalyst that produces an oxidizing agent (NO2) for oxidizing and eliminating the PM is separately provided upstream from the DPF, to thereby eliminate the PM in the DPF continuously without an external heat source.
Even with the continuous regeneration-type DPF, if the engine temperature is low, or the like, the PM trapped in the DPF cannot be completely eliminated depending on the operating conditions, resulting in the accumulation of the PM. Therefore, even if the continuous regeneration-type DPF is provided, there needs to be means, such as an external heat source, for forcibly incinerating the PM trapped in the DPF.
According to a method for directly incinerating the PM by an external heat source, such as the above burner, however, the DPF is directly heated by flame. Consequently, while this method has an easy and inexpensive configuration, there is a problem that there generates unevenness in the PM combustion and that the PM in the outer circumferential portion of the DPF cannot be satisfactorily eliminated. Moreover, in this case where the DPF is directly exposed to flame, there is great fear that the DPF is liable to be melted and damaged by being overheated.
In the case of a technology using the combustion heater described in the above patent document, the combustion heater is designed basically to raise the temperature of the cooling water of the engine. Therefore, heat loss is great for raising the temperature of the DPF. If the temperature rise of the DPF is prioritized, it creates a problem that the engine is applied with unnecessary heat load since the temperature of the cooling water is increased too much during the summer season, and the like, in which the outside air temperature is high. Moreover, the combustion heater is provided with a vaporizing glow plug besides an ignition glow plug to vaporize fuel by carrying electricity to the vaporizing glow plug during a non-operation period of the combustion heater, which complicates the device and greatly deteriorates the energy efficiency.